Chlorine as such, or in its various forms, is the main sterilization chemical used to control unwanted bacterial and algal growth in closed or semi-closed water reservoirs. The amount of chlorine required to maintain an effective microbicidal chlorine level in swimming pool water for example, depends in part on the chlorine demand of the water, e.g., from pollution brought in by the bathers. Additional factors that increase the chlorine demand are windblown dust, leaves and grass clippings, and other environmental contaminants.
There are a variety of ways to introduce chlorine into water. The most widely used method is addition of hypochlorites of sodium, calcium or lithium. Each of these salts has advantages and disadvantages in price and solubility. HOCl− generating chemicals such as di- and tri-chloro cyanuric acid are also used to produce hypochlorite.
Once in the water, equilibrium is established between the strong oxidant HClO− and the weaker ClO− ion. The equilibrium is pH dependent and is very sensitive in the range of pH 7 to 8. The negative charge on the hypochlorite ion hinders passage through bacterial membranes, so HOCl− is the preferred species to oxidize the cell contents of the bacteria. HOCl− is about 80 times more effective than the OCl− anion in killing microorganisms. Conversion of almost all free chlorine to HOCl− is easily accomplished by dropping the pH to about 6.
The use of HOCl− as the active biocide is limited by its sensitivity to heat and UV radiation of the sun, which results in relatively fast degradation into inactive species at ambient environmental conditions. These degrading conditions are heightened in the summer. The inclusion of cyanuric acid (CYA) in water treatment systems stabilizes and thus slows the degradation process of chlorine by sunlight, but it also shifts the equilibrium reaction to compensate for losses of OCl−. Therefore a continuous drop in the amount of the active form of chlorine occurs, and so additional chlorine salt must be continuously added.
As noted, CYA is beneficial for the protection of chlorine loss by UV radiation (in sunlight) at around 20-30 ppm. However, since most commonly used HOCl−-generating chemicals comprise di- and tri-chloro cyanuric acid, as the season progress, the level of the stabilizer (CYA) keeps rising with each cycle of replenishment of the sanitizer (HOCl−), which is constantly lost.
In certain situations, and as a result of over-stabilization, cyanuric acids are too high, and the chlorine is trapped and thus not effective as a disinfectant. This state, which is known as “chlorine-lock”, takes place when the concentration of cyanuric acid, which is rather stable, reaches over 100 ppm (corresponding to 0.77 mM). At this level the water is no longer safe for its original use, due a marked decrease in microbicidal efficacy of chlorine under “chlorine lock” conditions.
For these reasons, care is required to maintain relatively low levels of cyanuric acid in water purification contexts. Presently, this is achieved by the removal of cyanuric acid from the pool water either by dilution with fresh water or by emptying the pool, practices not only environmentally wasteful, but also generally impracticable for water reservoirs. Certain chemical procedures have been proposed for the removal of cyanuric acid but they too are unpractical (see for example, U.S. Pat. No. 4,793,935). Moreover, while CYA degradation can be accomplished enzymatically, the resultant CYA metabolites also build up in the water. Thus, there is a continuing need for methods to maintain low levels of cyanuric acid and its metabolites in water being treated with chlorine stabilizers.